1. Field of the Invention
The present invention relates to a method for producing diarylmethanes useful as various solvents, etc.
2. Description of the Related Art
Diarylmethanes are used as carbonless copying paper solvents because of being well soluble and compatible colorformers and dyes.
Heretofore, diarylmethanes have been prepared by the condensation of an alkylbenzene with an aldehyde at a low temperature using concentrated sulfuric acid as a catalyst, as disclosed in Japanese Patent Kokai Laid-open No. 61(1986)-27930. A problem with this method, however, is that the starting aldehyde itself is so liable to be polymerized that the reaction should be carried out at a very low temperature, say, -40 to -20C., needing a cooling medium. Also, the condensate should be neutralized to remove the sulfuric acid catalyst. Thus, this condensation method is not only troublesome to handle but is also low in both conversion and selectivity. Another long-established method, in which benzyl chloride is used as raw material in the presence of a Friedel-Crafts catalyst such as aluminium chloride or iron chloride, gives large amounds of by-products such as polymers of benzyl chloride and several compounds having 3 or more benzene rings. For instance, Japanese Patent Application Laid-Open No. 60(1985)-87231 discloses that chlorine is allowed to react with toluene through a free-radical reaction mechanism and an inorganic halide or acid is then permitted to act on the resulting reaction product. Japanese Patent Application Laid-Open No. 64(1989)-31734, on the other hand, set forth that chlorine is allowed to react with toluene or xylene in the presence of a free-radical generator and, after removal of the unreacted toluene or xylene, a benzene compound having a C.sub.2 -C.sub.16 hydrocarbon chain is allowed to react with the chlorinated product in the presence of an inorganic halide or acid. In the processes disclosed in both the publications, iron chloride, aluminium chloride and so on are used as the inorganic halides and concentrated sulfuric acid and zeolite as the inorganic acids. In their examples wherein only iron chloride is used, however, the yield of diarylmethanes is short of 80% or, in other words, 20% or more of six compounds having 3 or more benzene rings are formed. In still another method disclosed in French Patent No. 2543133, toluene is chlorinated to form dichlorotoluene and dichlorobenzyl chloride, which are then permitted to react with each other at 180.degree.-199.degree. C., using as a catalyst a cation-free X or Y type zeolite freed of cations, thereby forming tetrachloro-diphenylmethane.
As mentioned above, various methods for producing diarylmethanes from benzyl chlorides and alkylbenzenes have been known in the art. The method using the concentrated sulfuric acid or Friedel-Crafts catalysts, however, involves some problems in connection with the reaction temperature, by-products, reaction equipments and so on. The methods using the solid acid catalysts, on the other hand, have to be carried out at a reaction temperarure of at least 100.degree. C. and is less than satisfactory in terms of selectivity.
It is thus desired to develop a method for producing diarylmethanes under more mild conditions but more increased rectivity and selectivity than so far achieved